Method for characterizing a sample by nmr spectroscopy with acquisition during the generation of a radiofrequency magnetic field

ABSTRACT

The invention relates to a method for characterizing a sample ( 2 ) by means of a nuclear magnetic resonance spectrometer in which an effective field (B eff ) is generated, wherein said field has an effective vector (I) and results from a static magnetic field (B 0 ) and a radiofrequency magnetic field (B 1 ), characterized in that the effective vector (I) rotates relative to a terrestrial reference frame. The invention also relates to an apparatus ( 1 ) for implementing the method.

TECHNICAL FIELD

The invention relates to the field of methods for characterizing anobject (material, biological sample, or entire biological system in vivoor in vitro) by means of a nuclear magnetic resonance spectrometer. Theinvention also relates to the field of nuclear magnetic resonancespectrometry apparatuses for acquiring the magnetization of an object asdefined above.

PRIOR ART

In order to be able to study an object in nuclear magnetic resonance(NMR) spectrometry, it may be necessary to suppress or limit theinfluence of certain interactions occurring between the atoms making upthe object to be studied, for example the dipolar and/or quadrupolarinteraction between the nuclei of the atoms.

Suppression of these interactions between nuclei is generally carriedout by averaging.

For a liquid such as water contained in the large majority of biologicaltissues, the molecules move and are subject to rotations, randomly alongall the directions (this is designated as the Brownian movement). ThisBrownian movement is sufficiently rapid so that on average, the dipolarinteractions to which the atoms are subject are considered as equal tozero or relatively small.

This is why NMR spectrometry is relatively simple to set up for studyingliquid objects.

On the other hand, NMR spectrometry for studying a solid object is onlypossible provided that this object is placed under circumstancesallowing artificial recreation of the averaging effect of theinteractions so that from the point of view of NMR, the object may beassimilated to a liquid.

Indeed, in a solid, the Brownian movement is not sufficiently rapid toallow the dipolar and/or quadrupolar interactions perceived by thenuclei to equal to zero on average.

Notably, the non-zero dipolar interaction will make it possible for tothe nuclei of the constitutive atoms of the solid to interact with eachother and to distribute their spin in all the directions. The vector sum(magnetization) of the spins is then zero.

The cancelling of the magnetization occurs even more rapidly when thedipolar interaction is strong. Often, the dipolar interaction issufficiently strong for the magnetization to be cancelled out toorapidly to be usable (measurable).

Dipolar and quadrupolar interactions between two atom nuclei areproportional to 1-3.cos²(θ), wherein θ is an angle formed by an appliedstatic magnetic field and a vector connecting the centres of bothnuclei. Thus, in order to cancel out the effects of these interactions,it is sufficient that the average of 1-3.cos²(θ) may be considered asequal to zero. If the angle θ is on average equal to θ_(m)=arccos(3^(−1/2) or) π-arccos (3^(−1/2)) (i.e. approximately 54.74°) then theaverage of 1 3.cos² (θ) is zero. This angle θ_(m) is called a magicangle. In order to obtain an average of θ_(m) for the angle θ the solidmaterial is subjected to rapid rotation around an axis forming an angleθ_(m) with the vector of the static magnetic field required for NMRspectrometry. The faster the rotation, the more effective the averagingeffect of the interactions is and the closer one gets to the behavior ofa liquid sample and therefore the better the resolution of themeasurements is. The measurement is conducted in an Earth referencesystem which is that of the laboratory.

This spectroscopy technique adapted to solids is called Magic AngleSpinning (MAS).

A major drawback of this method is the requirement of rotating theobject. Within the scope of suppressing the dipolar interaction, inorder to obtain sufficient resolution, the rotation is performed at afrequency comprised between 1 Hz and 40 kHz. It is therefore notpossible to use this method in vivo, for studying human subjects forexample.

Further, these speeds of rotation even at a frequency of 40 kHz aresometimes still too slow for the quadrupolar interaction possiblypresent in the object to be considered as averaged to zero.

A second method, known from document US 2008/0,116,889, gives thepossibility of doing without the requirement of rotating the solidmaterial to be studied for “suppressing” the dipolar interaction.

In this second method, it is the main magnetic field (normally a staticfield) in which the material (equivalent to the static magnetic field ofthe previous method) is placed that undergoes rotation. Rotation of themain magnetic field is obtained by setting magnets into rotation. Therotation of the main magnetic field then creates the condition requiredfor cancelling out the average of 1-3.cos² (θ).

The major drawback of this method is the requirement of setting themagnets of heavy mass into rotation. Indeed, in a conventional NMRspectrometer, the magnet, whether this is a superconducting or resistivemagnet, enabling generation of the static magnetic fields, ischaracterized by a mass of several hundred kilograms or even severalmetric tons. It is then understood that even at frequencies of only afew hertz, setting parts of several metric tons into rotation involves ahighly significant and costly technological modification of present NMRspectrometers.

Another technique called Magic Echo Pulse Sandwich (MEPS) technique, maybe used on solid objects without requiring the use of mobile parts. Thislatter technique is based on the fact that, when, in addition to thestatic magnetic field B₀, a radiofrequency field B₁ is applied, thefield perceived by the atoms is an effective field B_(eff) for which thegeometrical characteristics are given by the equation:

${{\overset{->}{B}}_{eff} = {{\overset{->}{B}}_{1} + {\left( {B_{0} - \frac{v_{1}}{\overset{\_}{\gamma}}} \right)\hat{z}}}};$

wherein v₁ is the frequency of the applied radiofrequency field B₁,{right arrow over (B)}₀=B₀{circumflex over (z)}, and γ is thegyromagnetic ratio of the studied nucleus expressed in Hz·T¹ (γ=2.68·10⁸rad·Hz/T and

$\left. {\overset{\_}{\gamma} = \frac{\gamma}{2 \cdot \pi}} \right).$

When the frequency v₁, at which the field B₁ is applied, is equal toγ·B₀ (resonance frequency of the relevant atoms), the effective fieldB_(eff) perceived by the atoms is equal to the radiofrequency field B₁.It may be demonstrated that under these conditions, the dipolarinteraction perceived by the atoms is −½ times the dipolar interactionthat they perceive in the absence of the radiofrequency field B₁. Theprinciple of MEPSes gives the possibility of cancelling out the dipolarinteraction at a given instant. For this, during a portion of themeasurement the atoms are left to freely evolve for a duration τ, aduration during which they are subject to a dipolar interaction of valueH_(d). These atoms are then subject for a duration 2τ, to aradiofrequency field B₁ applied at frequency γ·B₀ so that they aresubjected during this duration 2τ to a dipolar interaction of value −½H_(d). Under these conditions, at instant 3τ, the magnetization istotally rid of the dipolar interaction (i.e. over the whole of theatoms, the influence of the dipolar interaction is zero, as if themagnetization had been subjected to no dipolar interaction). It is atthis instant 3τ that the signal is acquired.

This technique has the advantage of artificially rotating the effectivefield applied to the atoms thanks to the generated effective magneticfield B_(eff). It also has the advantage of recreating the conditionsfor cancelling out the dipolar and quadrupolar interaction.

On the other hand, this condition is only achieved at the single instant3τ; a single point of the signal then meets the condition of zerodipolar interaction. Now, if the dipolar and quadrupolar interactionsare significant, the measurements carried out a few microseconds aroundthe instant 3τ may already be too marred by the dipolar and/orquadrupolar interactions for their being utilized.

Thus, with this technique, in order to acquire a signal totally rid ofthe dipolar interaction, a point-by-point acquisition of this signal hasto be carried out for different values of the duration τ and by makingsure that for each acquired point at instant 3τ, the condition ofevolution, in which the atoms are in free evolution (i.e. subject toH_(d)) for a duration τ and in an evolution imposed by the static fieldB₀ (i.e. subject to −½ H_(d)) for a duration 2τ, is met.

The acquisition of a signal for a solid sample is therefore accomplishedpoint-by-point with this technique. It is therefore much slower (by anorder of a hundred to a thousand) than that using MAS.

SUMMARY

An object of the invention is to allow NMR spectrometry of solid objectsor objects including solid portions, in a rapid and non-destructive way.

To this aim, the invention provides a method for characterizing a sampleby means of a nuclear magnetic resonance spectrometer comprising anenclosure in which the sample is placed, a static magnetic fieldgenerator, a radiofrequency magnetic field generator, and at least onesensor, the method comprising the following steps:

-   -   generating in the enclosure, by the static magnetic field        generator, a static magnetic field with a static vector;    -   generating in the enclosure and for a determined duration, by        the radiofrequency magnetic field generator, a radiofrequency        magnetic field with a radiofrequency vector;

wherein the method further comprises the step for acquiring, through atleast one sensor, a magnetization of the sample for the determinedduration; and

wherein an effective vector, to which the magnetization of the sample issubjected for the determined duration, is rotating relatively to anEarth reference system for the determined duration, the effective vectorresulting from the static and radiofrequency magnetic fields, and thesample being fixed relatively to the Earth reference system.

An advantage of this method is that it is neither necessary to set thesample into rotation nor set the static magnetic field into rotation(equivalent to the main magnetic field). Thus, it is possible to conductin vivo studies of objects (for example animal or even human subjects)without requiring the modification of the enclosure housing one orseveral magnets which generate the static magnetic field.

Another advantage of this method is that the sample is placed underconditions for which the dipolar interaction is equal to zero for eachatom (and not only its influence on the totality of the sample) and thisfor the whole duration of the acquisition.

Other optional and non-limiting features are:

-   -   the radiofrequency magnetic field is modulated so that the        effective field applied to the sample has its effective vector        given by

${\overset{->}{B}}_{eff} = {{\overset{->}{B}}_{1} + {\left( {B_{0} - \frac{v_{1}}{\overset{\_}{\gamma}}} \right)\hat{z}}}$

and defining an angle (λ_(m)) of approximately 54.74° with the staticvector,

v₁ being a frequency close to the Larmor frequency v₀ due to the staticmagnetic field, this frequency v₁ being due to the radiofrequencymagnetic field;

-   -   the effective vector is rotating in a plane orthogonal to the        static vector;    -   the effective vector is rotating in a plane orthogonal to the        static vector at the Larmor frequency; and    -   acquisition of the magnetization of the sample is carried out by        at least two sensors; a first sensor placed so as to acquire        magnetic signals which are collinear with the static vector,        having a frequency close to the effective frequency, and at        least one second sensor placed so as to acquire magnetic signals        which are collinear to the plane orthogonal to the static        vector, having a frequency close to the Larmor frequency.

The invention also provides a nuclear magnetic resonance spectrometryapparatus for acquiring a magnetization of a sample comprising:

-   -   a sample holder fixed in an Earth reference system during the        operation of the apparatus;    -   an enclosure in which the sample holder is placed;    -   a static magnetic field generator for generating a magnetic        field with a static vector in the enclosure;    -   a radiofrequency magnetic field generator for generating a        radiofrequency magnetic field with a radiofrequency vector in        the enclosure for a determined duration;

wherein the apparatus further comprises at least one sensor formeasuring the magnetization of a sample for the determined duration.

Other optional and non-limiting features are:

-   -   the radiofrequency magnetic field generator comprises at least        two magnets or coils each generating a radiofrequency magnetic        field so that the effective magnetic field, resulting from both        radiofrequency magnetic fields of the magnets or coils and from        the static magnetic field, has a rotating effective vector in        the Earth reference system;    -   a first sensor is placed following the static vector and        adjusted so as to acquire signals at frequencies close to an        effective frequency resulting from an effective magnetic field        applied to the sample such that:

${{\overset{->}{B}}_{eff} = {{\overset{->}{B}}_{1} + {\left( {B_{0} - \frac{v_{1}}{\overset{\_}{\gamma}}} \right)\hat{z}}}};$

-   -   v₁ being a frequency close to the Larmor frequency due to the        static magnetic field, the frequency v₁ being due to the        radiofrequency magnetic field, and {right arrow over        (B)}₀=B₀{circumflex over (z)}; and

v _(eff) = γ·B _(eff),

γ being the characteristic gyromagnetic ratio of a studied atom nucleus;

-   -   two second sensors are placed in a plane orthogonal to the        static vector and adapted for acquiring a signal at a Larmor        frequency due to the static magnetic field with:

v ₀ = γ·B ₀,

γ being the characteristic gyromagnetic ratio of a studied atom nucleus;

-   -   the static magnetic field generator, the radiofrequency magnetic        field generator and the sensor(s) are fixed in the Earth        reference system during the operation of the apparatus.

PRESENTATION OF THE DRAWINGS

Other features, objects and advantages will become apparent upon readingthe detailed description which follows, with reference to the drawingsgiven as an illustration and not as a limitation, wherein:

FIGS. 1 a to 1 c are detailed illustrations of the effects of themagnetic fields customarily used in NMR spectrometry on the spinmagnetic moments of the studied atom nucleus;

FIG. 2 is a schematic illustration of a phenol molecule taken as anexample in the “principle of NMR spectrometry” part of the description,

FIG. 3 is a schematic illustration of an apparatus according to anembodiment of the invention,

FIG. 4 is a schematic illustration of a particular embodiment of theapparatus of FIG. 3;

FIG. 5 schematically shows an exemplary embodiment of the method of theinvention;

FIG. 6 is a time diagram showing the generated magnetic fields (at thetop, the radiofrequency magnetic field and at the bottom, the staticmagnetic field) as well as the acquisition step during the applicationof the method of FIG. 5;

FIG. 7 illustrates the decoupling of the surface antenna of the“butterfly” type and of the linear volumetric coil;

FIG. 8 illustrates an acquisition step as a time diagram accompanied bythe form of the acquired signals; and

FIG. 9 shows the different reference systems and their relationshipduring the application of the present invention.

DETAILED DESCRIPTION

Principle of NMR Spectrometry

The standard method for acquiring signals in NMR spectrometry isdescribed hereafter.

Generally, Nuclear Magnetic Resonance spectrometry (NMR spectrometry)consists of acquiring a signal proportional to the sum of the spinmagnetic moments of the atoms of an element contained in an objectplaced in a magnetic field, for example hydrogen ¹H atoms (thisspectrometry is then said to be proton NMR), deuterium atoms (²H),carbon 13 (¹³C) atoms, etc. Conventionally a spin magnetic moment isrepresented as a vector having a defined direction and a norm. Eachrelevant atom has a spin magnetic moment. The vector sum of the spinmagnetic moments of a material is called its magnetization.

In the absence of any applied magnetic field, the spin magnetic momentsof the studied atoms are randomly oriented in space. The magnetizationis then zero on average.

The magnetic field in which is placed the material consists of a staticmagnetic field with respect to an Earth reference system which is thatof the laboratory, and of a radiofrequency magnetic field. The staticmagnetic field is permanently applied onto the sample all along theexperiment. The radiofrequency magnetic field is applied in a pulsed way(i.e. briefly for a determined duration). The magnetization of theobject is measured in the absence of a radiofrequency field.

FIGS. 1 a to 1 c illustrate the known technique of NMR which consists ofgenerating a static magnetic field B₀ according to a static vector{right arrow over (B)}₀ continuously in an enclosure 11 in which anobject 2 of the studied material is placed, of generating aradiofrequency magnetic field B₁ as a radiofrequency pulse with aradiofrequency vector {right arrow over (B)}₁ for a determined durationT in the enclosure A2, and of acquiring the magnetization {right arrowover (M)} of the sample A1 after a predetermined evolution duration. Theamplitude of the static magnetic field B₀ is of the order of a teslawhile that of the radiofrequency magnetic field is at the very most ofthe order of a millitesla.

FIG. 1 a shows the influence of the static magnetic field B₀ on the spinmagnetic moments of the atoms (represented by vectors {right arrow over(S)} having as an origin a common point in space).

Under the action of the static magnetic field B₀, the angle formed byeach of the spin magnetic moments {right arrow over (S)} of the atomswith the static vector {right arrow over (B)}₀ is set and these spinmagnetic moments {right arrow over (S)} perform a precession movementaround the static vector {right arrow over (B)}₀. The link between theintensity of the static field B₀ and the frequency of rotation v₀ of themagnetizations is given by the relationship:

v ₀ = γ·B ₀;

γ being called the gyromagnetic ratio of the relevant nucleus expressedin Hz·T¹.

It may be demonstrated that under these conditions, the vector sum ofall the spin magnetic moments {right arrow over (S)} is then no longerzero on average. This sum of spin magnetic moments {right arrow over(S)} gives the magnetization {right arrow over (M)} of the materialwhich is on average non-zero and along the defined direction of thestatic vector {right arrow over (B)}₀.

FIG. 1 b shows the influence of the radiofrequency magnetic field B₁(represented by the radiofrequency vector {right arrow over (B)}₁,usually selected to be orthogonal to the static vector {right arrow over(B)}₀).

Under the action of the radiofrequency magnetic field B₁, themagnetization {right arrow over (M)} of the material performs a rotationaround the radiofrequency vector {right arrow over (B)}₁, the angle ofwhich is proportional to the intensity and to the determined generationduration T of the radiofrequency magnetic field B₁. Usually, theduration T is selected so that the angle of rotation is 90° (π/2) or180° (π). In an illustrative way, a rotation of 90° has been illustratedin FIG. 1 b.

The magnetization {right arrow over (M)} in fact performs a precessionmovement around the static vector {right arrow over (B)}₀ during therotation of 90° or 180°.

FIG. 1 c shows the evolution of the magnetization {right arrow over (M)}of the material after the determined duration T, while the staticmagnetic field B₀ continues to be generated and while the radiofrequencymagnetic field B₁ is no longer generated.

As soon as the radiofrequency magnetic field B₁ is no longer applied inthe enclosure 11, the magnetization {right arrow over (M)} will returnto the equilibrium state by loss of energy. This return to theequilibrium state is accomplished according to a precession movement(see the arrow F in FIG. 1 c) at a frequency specific to each of thespin magnetic moments {right arrow over (S)} composing the object.

The frequency of the magnetic signal generated by the return toequilibrium state of an atom is not the same for the same element if thelatter may be found in different electron environments. For example,phenol comprises 6 hydrogen atoms H_(a), H_(b), H_(c), H_(d), but thespin magnetic moments {right arrow over (S)} of each of the atoms willnot all return to the equilibrium state at the same frequency. There arefour groups of hydrogen atoms (see FIG. 2): the hydrogen atom H_(a)bound to the oxygen atom O, the hydrogen atoms H_(b) in the so-called“ortho” position, the hydrogen atoms H_(c) in the so-called “meta”position and the hydrogen atom H_(d) in the so-called “para” position.Each of the groups will experience its spin magnetic moment {right arrowover (S)} returning to the equilibrium state at a frequency differentfrom that of the other groups since their electron environments aredifferent. Nevertheless, these differences are minimal. In spite of thisthey may all be detected distinctly.

The total acquired signal is then a sum of the magnetic signals ofvarious frequencies. A Fourier transform may show these differentfrequencies, thereby forming an NMR spectrum of the sample.

Hereafter, only for the purposes of illustrations, proton NMRspectrometry (i.e. for hydrogen ¹H) will be taken as an example. Thisdoes not limit the invention to proton NMR spectrometry alone, but oneskilled in the art will easily be able to adapt the description whichfollows to NMR spectrometry of other atoms.

For the magnetic fields, {right arrow over (B)} represents the vector ofthe magnetic field and B represents the amplitude of the associatedvector and also refers to the magnetic field.

Apparatus

With reference to FIGS. 3 and 4, an NMR spectrometry apparatus 1 foracquiring a magnetization of a sample is described hereafter.

The NMR spectrometry apparatus 1 comprises a sample holder 12 forreceiving the sample 2. The holder 12 is intended to remain fixed in anEarth reference system during the operation of the apparatus 1.

The apparatus 1 also comprises an enclosure 11 in which the sampleholder 12 is placed. This enclosure 11 forms a volume in which magneticfields will be generated.

The apparatus 1 further comprises a static magnetic field generator 13for generating a static magnetic field B₀ in the enclosure 11 of staticvector {right arrow over (B)}₀. The static magnetic field generator 13is capable of generating a static magnetic field B₀ of an amplitude ofthe order of the tesla, typically between 0.1 T and 16 T. This staticmagnetic field B₀ gives the possibility of making the vector sum of thespin magnetic moments {right arrow over (S)} of the hydrogen atomsnon-zero along the line corresponding to the direction of the staticvector {right arrow over (B)}₀ without however making the spin magneticmoments {right arrow over (S)} individually collinear with the staticvector {right arrow over (B)}₀. The sum of the spin magnetic moments{right arrow over (S)} is the macroscopic magnetization {right arrowover (M)}, the latter is collinear with the static vector {right arrowover (B)}₀ and in the same defined direction. The spin magnetic moments{right arrow over (S)} form with the static vector {right arrow over(B)}₀ fixed angles and perform a precession movement around the staticvector {right arrow over (B)}₀ at the frequency of rotation v₀.

The apparatus also comprises a radiofrequency magnetic field generator14 for generating a radiofrequency magnetic field B₁ in the enclosure 11of radiofrequency vector {right arrow over (B)}₁. The static magneticfield generator 13 is capable of generating a radiofrequency magneticfield with an amplitude comprised between a few microteslas (μT) and afew milliteslas (mT), typically between 1 μT and 1 mT, which correspondsto frequencies comprised between about 40 Hz and 40 kHz for the proton.The radiofrequency magnetic field B₁ is generated, for a determinedduration T, as a pulse applied at the radiofrequency frequency v₁. Theradiofrequency field B₁ is modulated so that the resultant of the staticB₀ and radiofrequency B₁ magnetic fields as seen by the atoms rotates(see FIG. 9, where the laboratory reference system is labelled as (x, y,z), the reference system (x′, y′, z′) rotating at the angular frequencyω₁ around the axis {circumflex over (z)}, and the effective referencesystem (x_(eff), y_(eff), z_(eff)) rotating at the angular frequencyω_(eff) around the axis of the effective field {right arrow over(z)}_(eff), {right arrow over (z)}_(eff) being shifted by the magicangle relatively to the axis {circumflex over (z)}′ of the rotatingreference system). This resultant is called an effective field B_(eff)of effective vector {right arrow over (B)}_(eff) and defined by:

${{\overset{->}{B}}_{eff} = {{\overset{->}{B}}_{1} + \left( {{\overset{->}{B}}_{0} - \frac{{\overset{->}{v}}_{1}}{\overset{\_}{\gamma}}} \right)}};$

with γ the gyromagnetic ratio of the studied nucleus and the frequencyvector {right arrow over (v)}₁ collinear with the static vector {rightarrow over (B)}₀.

The radiofrequency magnetic field B₁ thus gives the possibility ofrotating the magnetization {right arrow over (M)} around the effectivevector {right arrow over (B)}_(eff).

The apparatus 1 comprises at least one sensor 15, 16 for acquiring themagnetization {right arrow over (M)} of the sample during the generationof the radiofrequency magnetic field B₁. This acquisition is carried outfor the determined duration T.

The apparatus 1 further comprises an actuator 17 for controlling thegeneration of the radiofrequency field B₁ in a sinusoidal way.

When the amplitude of the radiofrequency field B₁ as well as itsapplication frequency v₁ verifies the relationship:

${{\arctan\left( \frac{B_{1}}{B_{0} - \frac{v_{1}}{\overset{\_}{\gamma}}} \right)} = \theta_{m}};$$B_{1} = {\left( {B_{0} - \frac{v_{1}}{\overset{\_}{\gamma}}} \right) \cdot {{\tan \left( \theta_{m} \right)}.}}$

θ_(m) being the magic angle. The dipolar and quadrupolar interactionsperceived by the object are on average equal to zero and by acquiringthe signal during the application of the radiofrequency field B₁ with anamplitude and frequency defined above it is possible to get rid of therequirement of setting the sample 2 or the static magnetic fieldgenerator 13 into rotation. Thus, it is possible to study samplesregardless of their state (notably solid, but also liquid, gas state oreven a mixture thereof), or even study the tissues of an animal or of ahuman being in vivo.

The rotation frequency of the magnetization {right arrow over (M)} (dueto the rotating effective field B_(eff)) may be brought to a high value(of more than about a hundred kilohertz and optionally reaching onemegahertz) as compared with the MAS prior art method which is confinedto a few tens of kilohertz.

In an embodiment, the radiofrequency magnetic field generator 14consists of a single magnet or coil. The actuator 17 then controls theradiofrequency magnetic field generator 14 for generating aradiofrequency field B₁ sinusoidally modulated in order to rotate theeffective field B_(eff).

In another embodiment, the radiofrequency magnetic field generator 14may comprise at least two magnets or coils 141, 142 (see FIG. 4) eachgenerating a partial radiofrequency field. This gives the possibility ofobtaining a radiofrequency field B₁ for which the amplitude is moreintense. The actuator 17 then controls the generation of the partialradiofrequency magnetic fields by both coils 141, 142, so that theeffective field B_(eff) resulting from both partial radiofrequencymagnetic fields of the coils 141, 142 and from the static magnetic fieldB₀ has an effective vector {right arrow over (B)}_(eff) rotating aroundthe static vector {right arrow over (B)}₀ for the determined duration T.The partial radiofrequency magnetic fields are for example sinusoidallymodulated and in phase quadrature relatively to each other (whichamounts to having a partial radiofrequency magnetic field modulated by asine function and the other one by a cosine function).

During the application of the radiofrequency field B₁, the frequency v₁and the amplitude B₁ of which meet the condition:

${{\arctan\left( \frac{B_{1}}{B_{0} - \frac{v_{1}}{\overset{\_}{\gamma}}} \right)} = \theta_{m}};$

the spin magnetic moments {right arrow over (S)} and therefore theresulting magnetization {right arrow over (M)} perform a precessionmovement around the effective vector {right arrow over (B)}_(eff)defined by:

${\overset{->}{B}}_{eff} = {{\overset{->}{B}}_{1} + {\left( {B_{0} - \frac{v_{1}}{\overset{\_}{\gamma}}} \right)\hat{z}}}$

This precession movement takes place at the frequency v_(eff)=γ·B_(eff), The effective vector {right arrow over (B)}_(eff) of theeffective field B_(eff) is itself rotating around the static vector{right arrow over (B)}₀ at the Larmor frequency v₀= γ·B₀.

When the precession frequency around the effective field B_(eff) issufficiently large (i.e. of the same order as the frequencies used inMAS techniques, greater by a few hertz) an averaging of the dipolar andoptionally quadrupolar interaction present in the object is obtained.

The virtual rotation frequency v_(eff) responsible for the averaging ofthe dipolar and quadrupolar interactions being simply adjusted by theintensity of the applied radiofrequency field B₁, it is no longerneither the sample nor the static field B₀ which has to be set intorotation and tilted by the magical angle θ_(m) but only the effectivefield B_(eff), this latter operation being carried out by suitablyselecting the frequency v₁ and the amplitude of the radiofrequency fieldB₁. It is simple to give B₁ amplitudes ranging from a few μT up to onemT, these amplitudes generating a virtual rotation frequency v_(eff)ranging from a few Hz to several tens of kHz.

Thus, all the portions of the apparatus 1 remain fixed during itsoperation. Therefore this does not induce any mechanical wear of theapparatus 1 and considerably limits the risks of failures or of poorhandling.

A first sensor 15 may be placed along the static vector {right arrowover (B)}₀ and be adjusted so as to detect frequencies around theeffective frequency v_(eff). The effective frequency v_(eff) is due tothe effective magnetic field of B_(eff) seen by the sample 2. Theeffective magnetic field B_(eff) results from the combination of thestatic B₀ and radiofrequency B₁ magnetic fields (without taking intoaccount the electron environment of the hydrogen atom), and has theformula given by the following relationship:

${\overset{->}{B}}_{eff} = {{\overset{->}{B}}_{1} + {\left( {B_{0} - \frac{v_{1}}{\overset{\_}{\gamma}}} \right)\hat{z}}}$

The frequency v₁ is the application frequency of the field B₁, this is afrequency close to the Larmor frequency of the studied nucleus v₀= γB₀,

The following relationship gives the effective frequency v_(eff):

v _(eff) = γ·B _(eff).

Thus, the first sensor 15 allows acquisition of the signals frequenciesclose to the effective frequency v_(eff) which correspond to the signalsresulting from the precession movement of the spin magnetic moments{right arrow over (S)} of the hydrogen atoms of the sample 2 around theeffective field B_(eff).

Two second sensors 16 may be placed in a plane orthogonal to the staticvector {right arrow over (B)}₀ and adapted so as to acquire signals atthe Larmor frequency v₀.

The sensor(s) 15, 16 are decoupled from the generator(s) 14; 141, 142 ofthe radiofrequency magnetic field B₁. This decoupling may be achievedgeometrically or electronically. An example is given below.

The use of the first and second sensors 15, 16 gives the possibility ofacquiring the variations of the magnetization {right arrow over (M)}along the three spatial dimensions in the reference system of thelaboratory.

Generally, the configuration of the sensors may be selected from thefollowing configurations:

-   -   a single sensor for acquiring signals at a frequency close to or        equal to the Larmor frequency v₀;    -   two orthogonal sensors for acquiring signals at a frequency        close or equal to the Larmor frequency v₀;    -   two sensors, one of which for acquiring signals at a frequency        close or equal to the Larmor frequency v₀; and one for acquiring        signals at a frequency close or equal to the effective frequency        v_(eff);    -   three sensors, two of which for acquiring signals close or equal        to the Larmor frequency v₀ and one for acquiring signals at a        frequency close or equal to the effective frequency v_(eff).

Method

With reference to FIGS. 5 and 6, a method for characterizing a sample bymeans of an NMR spectrometry apparatus described above, is describedhereafter.

Prior to the method, a sample 2 is placed on the holder 12 of the NMRapparatus 1, inside the enclosure 11.

The method comprises generating E1 in the enclosure 11 of the apparatus1, by the static magnetic field generator 13, a static magnetic field B₀of static vector {right arrow over (B)}₀ collinear with a unit vector{circumflex over (z)}.

The method also comprises generating E2 in the enclosure 11, by theradiofrequency magnetic field generator 14, a radiofrequency magneticfield B₁ of radiofrequency vector {right arrow over (B)}₁. Theradiofrequency magnetic field B₁ is generated for a determined durationT, as a pulse I.

The amplitude B₁ of the radiofrequency magnetic field B₁ of frequency v₁is given by the relationship:

${{\arctan\left( \frac{B_{1}}{B_{0} - \frac{v_{1}}{\overset{\_}{\gamma}}} \right)} = \theta_{m}};$

θ_(m) being the magic angle.

The radiofrequency magnetic field B₁ is, for the determined duration T,modulated so that an effective field B_(eff) seen by the atoms of theobject results from the static B₀ and radiofrequency B₁ magnetic fields,is of an effective vector {right arrow over (B)}_(eff) rotatingrelatively to an Earth reference system, such that:

${\overset{->}{B}}_{eff} = {{\overset{->}{B}}_{1} + {\left( {B_{0} - \frac{v_{1}}{\overset{\_}{\gamma}}} \right){\hat{z}.}}}$

The effective vector {right arrow over (B)}_(eff) of the effective fieldB_(eff) performs a precession movement around the static vector {rightarrow over (B)}₀ at frequency v₀. The radiofrequency field B₁ wasselected so that the effective vector {right arrow over (B)}_(eff) formsa magic angle of θ_(m)=54.74° with the static vector {right arrow over(B)}₀.

The radiofrequency magnetic field B₁ may only comprise a singlesinusoidally modulated component. In this case, only the effectivevector {right arrow over (B)}_(eff) rotates in the Earth referencesystem.

The radiofrequency magnetic field B₁ may also comprise two sinusoidallymodulated components in phase quadrature. In this second case, theradiofrequency and effective {right arrow over (B)}_(eff) vectors rotatein an Earth reference system.

By applying the effective field B_(eff) forming the magic angle θ_(m)with the static field of B₀, it is possible to get rid of therequirement of setting the sample 2 or the static magnetic fieldgenerator 13 into physical rotation. Thus, it is possible to studysamples regardless of their state (notably solid, but also liquid, gasstate or even a mixture thereof), or even study the tissues of an animalor of a living human being.

Further, the rotation frequency of the magnetization {right arrow over(M)} (due to the rotating effective field B_(eff) and proportional tothe effective frequency v_(eff)) may be raised to a high value, whichmay exceed one megahertz, as compared with the prior art which islimited to a few tens of kilohertz.

The experimenter selects the virtual rotation speed, proportional to theeffective frequency v_(eff), which is intended to be imposed to thesample. Once this effective speed of rotation is selected (between a fewhertz and one megahertz), the amplitude of the radiofrequency field B₁as well as its frequency v₁ are found by solving the system ofequations:

$\quad\left\{ \begin{matrix}{B_{1}^{2} + \left( {B_{0} - \frac{v_{1}}{\overset{\_}{\gamma}}} \right)^{2} - B_{eff}^{2}} \\{{B_{1} = {\sqrt{2} \cdot \left( {B_{0} - \frac{v_{1}}{\overset{\_}{\gamma}}} \right)}};}\end{matrix} \right.$

with B₀, B_(eff) and γ being known.

The method further comprises a step E3 for acquiring by at least onesensor 15, 16 a magnetization {right arrow over (M)} of the sample 2.This acquisition step E3 is carried out for the determined duration Tand lasts for an acquisition time T_(a) comprised in the determinedduration T (see FIG. 6).

For example, two decoupled antennas (or coils) are used: a first antenna14 is a linear volumetric coil (for example a Rapidbiomedical V-HLS-047model) for emitting the radiofrequency magnetic field and a second one15 is a surface antenna of the “butterfly” type for reception.

The decoupling of both coils 14, 15 is achieved according to two steps.First of all, a rotation of angle ∝_(optimum) is applied to the plane ofthe butterfly antenna in order to make the butterfly antenna as parallelas possible to the radiofrequency vector {right arrow over (B)}₁generated by the emitting antenna 14. This minimizes the flux of theradiofrequency magnetic field B₁ through both loops of the butterflyantenna to a residual flux. And then, the butterfly antenna istranslationally displaced inside the emitting antenna 14. Thereby makingthe most out of the non-uniform nature of the radiofrequency magneticfield B₁, it is possible to find a position in which the flux differencebetween both loops 151, 152 of the butterfly antenna 15 may cancel outthe residual flux.

Moreover, in this case, the sample 2 may be positioned inside a firstloop 151 of the butterfly antenna (see FIG. 7). Thus, the flux producedby the magnetization of the sample is maximum in the first loop 151 andalmost equals to zero in the second loop 152.

Thus, by means of this method, it is possible to get rid of therequirement of conducting measurements point-by-point as imposed in theMEPS technique.

The acquisition step E3 may comprise the substep E31 for acquiringmagnetic signals having a frequency close to the effective frequencyv_(eff). This low frequency signal gives information on thetime-dependent change of the magnetizations according to the axis of thestatic field B₀.

The acquisition step E3 may also comprise the substep E32 for acquiringmagnetic signals having a frequency close to the Larmor frequency v₀.With this step it is possible to monitor changes of the signal over timein the plane perpendicular to the static vector {right arrow over (B)}₀.

The signals collected by the sensors 15, 16 may then be filtered E4 inorder to remove parasitic signals from the generators 13, 14; 13, 141,142 of magnetic fields.

The connected signals are then processed E5 for reconstructing athree-dimensional signal describing over time the changes of themagnetization M of the sample 2. The three-dimensional signal may be putinto the form of quaternion signal i.e. in the form of a signal forwhich each point is put into the form of a+ib+jc+kd with a, b, c, dbeing real and i²=j²=k²=ijk=−1. At each instant t of the acquisition,the three-dimensional signal may be described in spherical coordinatesby three parameters (ρ, θ, φ) corresponding to the amplitude, thelatitude and colatitude of the signal. Finally a demodulation step E6may be carried out on the three-dimensional signal in order to representit in the Earth reference system according to a fixed reference system.

FIG. 8 illustrates a time diagram showing the acquisition of the signalsduring the emission of the radiofrequency magnetic field B₁. Accordingto this particular time diagram, the acquisition is triggered beforeapplication of the radiofrequency magnetic field B₁ and the acquisitionends after having cut off the radiofrequency magnetic field B₁.

FIG. 8 also shows both acquired signals. The first at the topcorresponds to the real part of the transverse magnetization while thesecond at the bottom corresponds to the complex part of thismagnetization. Both oscillating and observable signals persist for aperiod of more than 300 ms, which is a relatively long time in the fieldof NMR acquisition. When the radiofrequency magnetic field is cut off,the observed signal disappears within only about 20 milliseconds.

1. A method for characterizing a sample (2) by means of a NuclearMagnetic Resonance spectrometer comprising an enclosure (11) in whichthe sample (2) is placed, a static magnetic field (B₀) generator (13), aradiofrequency magnetic field (B₁) (14), and at least one sensor (15,16), the method comprising the following steps: (a) generating (E1) inthe enclosure (11), by the static magnetic field generator (13) a staticmagnetic field (B₀) with a static vector ({right arrow over (B)}₀); (b)generating (E2) in the enclosure (11) and for a determined duration (T),by the radiofrequency magnetic field generator (14) a radiofrequencymagnetic field (B₁) with a radiofrequency vector ({right arrow over(B)}₁); wherein it further comprises the step (c) for acquiring (E3) byat least one sensor (15, 16), a magnetization ({right arrow over (M)})of the sample (2) for all or part of the determined duration (T); andwherein an effective vector ({right arrow over (B)}_(eff)), to which themagnetization ({right arrow over (M)}) of the sample (2) is subjectedfor the determined duration (T), is rotating relatively to an Earthreference system for the determined duration (T), the effective vector({right arrow over (B)}_(e)) resulting from the static (B₀) andradiofrequency (B₁) magnetic fields, and the sample (2) being fixedrelatively to the Earth reference system.
 2. The method according toclaim 1, wherein the radiofrequency magnetic field (B₁) is selected sothat the effective field (B_(eff)) applied to the sample has itseffective vector ({right arrow over (B)}_(eff)), given by${\overset{->}{B}}_{eff} = {{\overset{->}{B}}_{1} + {\left( {B_{0} - \frac{v_{1}}{\overset{\_}{\gamma}}} \right)\hat{z}}}$and defining an angle (θ_(m)) of approximately 54.74° with the staticvector ({right arrow over (B)}₀) v₁ being a frequency close to theLarmor frequency v₀ due to the static magnetic field (B₀), thisfrequency v₁ being due to the radiofrequency magnetic field (B₁).
 3. Themethod according to one of claims 1 and 2, wherein the effective vector({right arrow over (B)}_(eff)) is rotating in a plane orthogonal to thestatic vector ({right arrow over (B)}₀).
 4. The method according toclaims 2 and 3, wherein the effective vector ({right arrow over (B)}₁)is rotating in a plane orthogonal to the static vector (B₀) at theLarmor frequency (v₀).
 5. The method according to claim 4, wherein theacquisition (E3) of the magnetization ({right arrow over (M)}) of thesample (2) is carried out by at least two sensors (15, 16); a firstsensor (15) placed so as to acquire magnetic signals collinear with thestatic vector ({right arrow over (B)}₀), having a frequency close to theeffective frequency (v_(eff)), and at least one second sensor (16)placed so as to acquire magnetic signals collinear with the planeorthogonal to the static vector ({right arrow over (B)}₀), having afrequency close to the Larmor frequency (v₀).
 6. A Nuclear MagneticResonance spectrometry apparatus for acquiring a magnetization ({rightarrow over (M)}) of a sample (2) comprising: a sample holder (12) fixedin an Earth reference system during the operation of the apparatus (1);an enclosure (11) in which the sample holder (12) is placed; a staticmagnetic field generator (13) for generating a static magnetic field(B₀) with a static vector ({right arrow over (B)}₀) in the enclosure(11) a radiofrequency magnetic field generator (14) for generating aradiofrequency magnetic field (B₁) with a radiofrequency vector ({rightarrow over (B)}₁) in the enclosure for a determined duration (T);wherein the apparatus (1) further comprises at least one sensor (15, 16)for measuring the magnetization ({right arrow over (M)}) of the sample(2) for the determined duration (T).
 7. The apparatus (1) according toclaim 6, wherein the radiofrequency magnetic field generator (14)comprises at least two coils (141, 142) each generating a radiofrequencymagnetic field so that the resulting total effective magnetic field(B_(eff)) resulting from both radiofrequency magnetic fields of thecoils (141, 142) and from the static magnetic field (B₀) has aneffective vector ({right arrow over (B)}_(eff)) rotating in the Earthreference system.
 8. The apparatus (1) according to one of claims 6 and7, wherein a first sensor (15) is placed according to the static vector({right arrow over (B)}₀) and adjusted so as to acquire signals atfrequencies close to an effective frequency (v_(eff)) resulting from aneffective magnetic field (B_(eff)) applied to the sample (2) such that:${{\overset{->}{B}}_{eff} = {{\overset{->}{B}}_{1} + {\left( {B_{0} - \frac{v_{1}}{\overset{\_}{\gamma}}} \right)\hat{z}}}};$v₁ being a frequency close to the Larmor frequency (v₀) due to thestatic magnetic field (B₀), the frequency v₁ being due to theradiofrequency magnetic field (B₁); andv _(eff) = γ·B _(eff), γ being the gyromagnetic ratio characteristic ofa nucleus of a studied atom.
 9. The apparatus (1) according to one ofclaims 6 to 8, wherein two second sensors (16) are placed in a planeorthogonal to the static vector ({right arrow over (B)}₀) and adaptedfor acquiring a signal at a Larmor frequency (v₀) due to the staticmagnetic field (B₀) with:v ₀ ={right arrow over (γ)}·B ₀, γ being the gyromagnetic ratiocharacteristic of a nucleus of a studied atom.
 10. The apparatus (1)according to one of claims 6 to 9, wherein the static magnetic fieldgenerator (13), the radiofrequency magnetic field generator (14) and thesensor(s) (15, 16) are fixed in the Earth reference system during theoperation of the apparatus (1).